In service, a microwave transmitter reflector (also called the main reflector or the antenna) is aimed at a distant location of interest. The microwave transmitter reflector either receives microwave signals from that distant location, or transmits microwave signals toward that distant location. A high-gain microwave transmitter reflector system typically has a dish-type microwave transmitter reflector that is pointed at the distant location for reception and transmission. The pointing is accomplished by mounting the microwave transmitter reflector on a gimbal structure that permits aiming in both the elevation and azimuth orientations. The structures of the microwave transmitter reflector and gimbal are desirably made no heavier than necessary to avoid an overly large gimbal structure.
Reflector-type microwave antenna systems are subject to electromechanical, optical, and/or mechanical misalignments. The result of the misalignments is a mispointing and possible off-axis aberrations imparted to the main microwave transmitted beam or incoming microwave signal. The highest-gain microwave antennas have large dish-type microwave transmitter reflectors, and even slight misalignments can greatly decrease the performance of the antenna system, resulting in the loss of key data or the arrival of less power.
One of the principal sources of misalignment is the tolerances and deformation associated with the mechanical elements of the microwave transmitter reflector. Mechanical tolerances in all parts of the gimbal assembly, such as gear backlash, tilts of relay optics, and bearing wear and tolerances, cause beam misalignment, when the transmitter source is not on the gimbal assembly (as is usually the case to keep the gimbaled weight as low as possible). The amount of the mechanical misalignments varies with the pointing angle of the gimbal, as the weight-of the microwave transmitter reflector shifts. The mechanical misalignment also varies with the service age of the antenna system, since the mechanical wear increases over time. Particularly for microwave systems operating in the higher frequencies, such as high-gigahertz-frequency systems where the wavelength is on the order of millimeters, the deformation and mechanical errors may be a significant portion of a wavelength. In such instances, the mispointing of the microwave transmitter reflector as it is pointed in different directions can result in a significant misalignment and loss of signal or power level. Other sources of misalignment are the mechanical deforming of the reflective elements of the system, such as the microwave transmitter reflector and the microwave mirrors, and pointing errors due to extraneous factors such as gusty wind loadings.
For some microwave systems, such as those using fixed, ground-based high-gain reflector-type antennas, the misalignment may be calibrated so that the pointing of the reflector is corrected as a function of the pointing angle. Other types of errors, such as wind loading, gear backlash, bearing wear, and differential thermal expansion, cannot be corrected through the calibration approach.
In these other cases, alternative approaches, such as using a visible laser aiming system operating in conjunction with the microwave aiming system, may be used. See, for example, U.S. Pat. No. 6,252,558, whose disclosure is incorporated by reference. Such approaches are highly successful for some applications. In others, the use of a visible laser of sufficiently high power raises eye-safety and visibility concerns, and also requires the optical elements to have optical quality surfaces. Additionally, this approach does not address the fundamental problem of beam skew due to asymmetrical incident phase contours.
There is a need for an improved approach to correcting the aiming of a microwave antenna reflector. The present invention fulfills this need, and further provides related advantages.